In order to realize high-density recording on a large-capacity optical disc, for example, a Blu-ray Disc (BD), it is necessary to reduce a light spot diameter of laser light irradiated onto an information recording surface of the optical disc. In order to make the light spot diameter small, it is necessary to shorten a wavelength of the laser light and to increase a numerical aperture (NA) of an objective lens, but this leads to increase of a spherical aberration. The spherical aberration is proportional to the fourth power of NA of the objective lens and is inversely proportional to a wavelength of a light flux. For this reason, a spherical aberration occurring when BD (wavelength of 405 nm; NA of 0.85) is reproduced is approximately 6.5 times (≈(0.85/0.6)4×(650/405)) in comparison with a spherical aberration occurring when DVD (wavelength of 650 nm; NA of 0.6) is reproduced. If the spherical aberration increases, a light spot of laser light irradiated onto the information recording surface of the optical disc changes in shape and reproduction performance is deteriorated. A value of the spherical aberration varies depending on a thickness error of a layer disposed on the information recording surface of the optical disc. Accordingly, in order to maintain high reproduction performance, it is important to perform adjustment so as to reduce the spherical aberration as much as possible.
Furthermore, the light spot of the laser light irradiated onto the information recording surface of the optical disc also varies in shape depending on performance of focus servo which makes the objective lens follow an optimum position in a direction perpendicular to the information recording surface of the optical disc. An appropriate adjustment of an offset value (focus offset) of the focus servo enables to improve the focus servo performance and to have an appropriate shape of the light spot on the information recording surface of the optical disc. Further, a value of the focus offset varies depending on a thickness error of the layer on the information recording surface of the optical disc. Accordingly, in order to improve the focus servo performance and maintain high reproduction performance, it is important to adjust the focus offset.
For example, Patent Document 1 discloses that characteristics of an amplitude value and a jitter value of a reproduction signal are represented by a concentric area whose center is an optimum amplitude value of the reproduction signal and a concentric area whose center is an optimum jitter value respectively, in a two-dimensional map whose horizontal and vertical axes represent a spherical aberration and a focus offset respectively, and discloses a method of adjusting a spherical aberration and a focus offset.
Further, Patent Document 2 discloses a method of adjusting a spherical aberration and a focus offset so that an amplitude value of a tracking error signal is the maximum.
Furthermore, Patent Document 3 discloses a method of causing a spherical aberration correcting means to be a set state that a focus loop gain value is maximum in a first process, supplying an offset value which makes an amplitude value of a tracking error signal maximum to a focus actuator in a second process, and then implementing the first process again.
Moreover, Patent Document 4 describes a method of measuring focus offset dependencies of a first evaluation indicator indicating information track crossing signal quality grade and a second evaluation indicator indicating information reproduction signal quality grade, at least two or more spherical aberration correction values, determining correlation between spherical aberration correction values of the first and second evaluation indicators and a focus offset optimum point as first and second polynomial approximation curves respectively, and setting an intersection of these first and second polynomial approximation curves as a desirable spherical aberration correction value and a focus offset value.